Methods and apparatuses for performing preamble assignment for random access in a telecommunications system

ABSTRACT

The present invention relates to methods and apparatuses (radio base station ( 120, 600 ) and UE ( 110, 700 ) for enabling a UE to perform a random access. According to embodiments of the present invention, an identification number of a dedicated random access preamble is, in a message, transmitted together with information indicating in which channel or channels the preamble is valid for the UE. The UE ( 110, 700 ) can then use the received information and based on that information perform a random access.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/619,766, filed Jun. 12, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/185,480, filed Jun. 17, 2016, which is acontinuation of U.S. patent application Ser. No. 13/947,191, filed Jul.22, 2013, which is a continuation of U.S. patent application Ser. No.13/002,314, filed Dec. 31, 2010, which is a National Stage of PCTApplication No. PCT/SE2008/051534,filed Dec. 19, 2008, which is relatedto, and claims priority from, U.S. Provisional Patent Application Ser.No. 61/077,295 filed Jul. 1, 2008, the disclosure of which isincorporated here by reference.

TECHNICAL FIELD

The present invention relates generally to the field of wirelesstelecommunications, and, more particularly, to methods and apparatusesfor assignment of a preamble to a user equipment for enabling the userequipment to perform a random access in a telecommunications system.

BACKGROUND

The 3rd Generation Partnership Project (3GPP) is responsible for thestandardization of the UMTS (Universal Mobile Telecommunication Service)system, and LTE (Long term Evolution) is now under discussion as a nextgeneration mobile communication system of the UMTS system. LTE is atechnology for realizing high-speed packet-based communication that canreach data rates of more than 100 Mbps on the downlink and of more than50 Mbps on the uplink. The 3GPP work on LTE is also referred to asEvolved Universal Terrestrial Access Network (E-UTRAN).

Generally, one or more cells are allocated to a radio base station,known in the 3GPP LTE system as eNB (enhanced/evolved NodeB) or eNodeB.In addition, eNBs in LTE will interact directly with the core networkand with other eNBs. A plurality of user equipments can be placed in acell served by an eNB. A user equipment (UE) can be represented by amobile phone, a wireless terminal, a laptop, a personal computer, apersonal digital assistant, a voice over internet protocol (VoIP)capable phone or any other 3GPP LTE capable UE. Generally, a UE's firstaccess to the system is performed by means of a random access (RA)procedure. The objectives of the RA procedure may include: initialaccess; handover; scheduling request (request for radio resources);timing synchronization, and the like. The radio network nodes generallycontrol the behavior of the UE. As an example, uplink transmissionparameters like frequency, timing and power are regulated via downlinkcontrol signalling from the radio base station (e.g. eNB) to the UE. Forthe uplink (UL) frequency and power estimate parameters, a UE can derivethose parameters from one or several downlink (control) signals.However, making a timing estimate for the uplink is more difficult dueto that the propagation delay between the eNB (or eNodeB) and the UE isgenerally unknown. As an example, when a UE is powered on or turned onor after a long standby time, the UE is not synchronized in the uplink.Therefore, before commencing traffic, the UE has to access the network,which in a first step includes obtaining synchronization to the network.This is usually done by the UE which performs measurement(s) bylistening to downlink signals and obtain from these signals timingsynchronization; an estimate of a frequency error, and also an estimateof the downlink pathloss. Even though the UE is now time-synchronized tothe downlink, signals to be sent from the UE are still not aligned withthe reception timing at the eNB (or eNodeB) due to said unknownpropagation delay. Thus the UE has to carry out a random access (RA)procedure to the network. The RA procedure is a procedure typically usedby the UE to request access to a system or resources when the UEdiscovers a need to acquire uplink synchronization or a need to make anuplink transmission and no resources for said uplink transmissions areyet available to the UE. Furthermore, synchronization or time alignmentof uplink transmissions aims to minimize interference with transmissionsof other UEs and increase resource efficiency by minimizing the need forguard bands.

The RA procedure can be classified into a contention-based random accessprocedure and a contention-free (or non-contention-based) random accessprocedure.

For the contention-based random access procedure, a first set forming apool of non-dedicated random access preambles is assigned per cell (i.e.to a eNodeB). This pool is primarily used when there is UE-originateddata and the UE has to establish a connection and an adequate uplinktiming relation with the network through the RA procedure. Whenperforming contention-based random access, the UE arbitrarily selects apreamble from the pool as the non-dedicated random access preamble. Thisis known as UE initiated random access (supported in LTE). Thus forcontention-based random access, the network (or the eNB) is not(immediately) aware of which UE selected which preamble. A drawback withthis is that multiple UEs may in fact select the same preamble and theymay attempt to access the network (or eNodeB) at the same time. This maycause collision(s) to occur. Thus, an extra step of identifying UEstrying to access the network (or eNodeB) and resolving potentialcollisions, a so-called contention resolution mechanism, is needed.

For performing contention-free random access, there is also defined asecond set forming a pool of random access preambles assigned per cell(i.e. to a eNodeB). These preambles are known as dedicated random accesspreambles. Contrary to the non-dedicated random access preambles, adedicated random access preamble is assigned to the UE by the eNodeB. Inother words, this preamble cannot be autonomously selected by the UE andtherefore, for the duration of the validity of the assignment, thisdedicated random access preamble is exclusively dedicated to the UE.This is known as network triggered or network ordered random access(supported in LTE). Since a specific preamble is assigned/dedicated tothe UE, it is a benefit of contention-free access that the eNodeB canimmediately know from the received preamble, which UE tries (or tried)to access the network. This thus eliminates the need for contentionresolution and therefore improves resource efficiency by minimizing therisk of collisions. Furthermore, avoiding the contention resolutionprocedure reduces the delay.

It should be noted that the network-triggered (e.g. in the E-UTRAN)random access can be used to force a UE, which does not have a validuplink timing to synchronize its uplink to the timing at the eNodeB,e.g., prior to the eNodeB making a downlink transmission for which theUE will need to transmit an acknowledgment (ACK) or anegative-acknowledgment (or a non-acknowledgment) (NACK) feedback. Itshould also be mentioned that because of the non-zero duration of therandom access and uplink synchronization procedure, there-synchronization is typically forced in advance of making the downlinktransmission. In general a UE (e.g. a mobile terminal) loosessynchronization if it is not active for a certain time. It is thuspossible to define a decision criterion in the network (e.g. in theE-UTRAN) that after a certain time without any activity a UE (e.g. amobile terminal) is assumed to have lost synchronization to the eNB.

If e.g. downlink data arrives at the UE, synchronization needs to bere-established first, and for this purpose, the above describeddedicated random access preamble is assigned to the UE, which the UE canuse to perform a contention-free random access procedure. This willtrigger a timing advance adjustment command from the eNB, and based onthis command, the UE can re-establish time-alignment. For instance, inLTE (or E-UTRAN) the physical downlink control channel PDCCH is used toassign a dedicated preamble.

Since the random access procedure is the first procedure performed bythe UE to access the network, it is important that random access worksas it should. If random access fails, the UE cannot access the network.An exemplary scenario where a random access procedure can fail or cannotbe performed is when all of the dedicated preambles are already in use.As mentioned before, the eNodeB keeps track of the dedicated preamblesit has already allocated/assigned, and when there is no such dedicatedpreamble available for allocation, the eNB has to advice/refer otherUEs, for which there is no dedicated random preamble, to perform acontention-based random access. However, as mentioned before,contention-based random access leads to potential collisions andadditional delays when a contention resolution needs to be performed. Ifcollisions occur and one or more UEs fail with their random access, oneor several UEs may retry the random access procedure after a certaintime which leads to that a back-off mechanism may be triggered thatintroduces further delays. A back-off mechanism is an approach tocontrol UE re-accesses attempts by introducing e.g. a back-off parameterthat represents a period of time to wait before an access reattempt.Thus, the larger the number of re-access attempts, the longer the delay.

SUMMARY

It is therefore an object of the exemplary embodiments of the presentinvention to address the above mentioned problems and to provide methodsand apparatuses for enabling one or more UEs to perform a successfulrandom access by allowing the use of one or more dedicated random accesspreamble(s) for one or more UEs by introducing a validity patternrepresenting in which channel occurrence(s) the dedicated preamble(s) isvalid to be used by UE(s) for random access. This will lead to areduction in random access delays and also a reduction in the number ofcollisions that may occur.

According to a first aspect of embodiments of the present invention, theabove stated problem is solved by means of a method in a radio basestation of assigning a preamble to a UE. The radio base stationrepresenting a eNB or a eNodeB or a NodeB, is allocated a first setforming a pool of non-dedicated random access preambles and a second setforming a pool of dedicated random access preambles. According toembodiments of the present invention, the radio base station determinesan available dedicated random access preamble from the second set; itthen transmits a message to the UE, the message comprising anidentification number of the available dedicated preamble and furthercomprising information indicating in which of one or more PRACH(physical random access channel) occurrences, the dedicated preamble isvalid to be used by the UE for random access.

Thus, in order to, for example, to alleviate the case where there isshortage of dedicated random access preambles and in order to extent theavailability of dedicated random access preambles, the radio basestation is configured to determine the availability of a dedicatedrandom access preamble for assignment to the UE and to transmit in amessage, the identification number of the dedicated preamble and furtherindicating in said message a validity pattern for enabling the UE toknow in which PRACH occurrence(s) the dedicated preamble is valid forthe UE.

According to a second aspect of embodiments of the present invention,the above stated problem is solved by means of a method in a UE toenabling said UE to perform a random access in a telecommunicationssystem comprising a radio base station to which, as mentioned above, isallocated a first set forming a pool of non-dedicated random accesspreambles and a second set forming a pool of dedicated random accesspreambles. The method according to embodiments of the present inventioncomprises the steps of: receiving a message from the radio base station,the message comprising an identification number of an availablededicated access preamble of the second set. The message furthercomprising information indicating in which of one or more PRACHoccurrences the dedicated random access preamble is valid to be used bythe UE for random access. The method further comprises: performing arandom access based on the received identification number of theavailable dedicated preamble and based on the indicated informationconcerning said one or more PRACH occurrences.

According to a third aspect of embodiments of the present invention, theabove stated problem is solved by means of a radio base station forassigning a preamble to a UE for enabling the UE to perform randomaccess. As mentioned above, a first set forming a pool of non-dedicatedrandom access preambles and a second set forming a pool of dedicatedrandom access preambles are assigned to the radio base station. Theradio base station, according to embodiments of the present invention isconfigured to determine a dedicated random access preamble from thesecond set; to transmit a message to the UE, the message comprising anidentification number of the determined dedicated preamble andcomprising information indicating in which of one or more PRACH(physical random access channel) occurrences, the dedicated preamble isvalid to be used by the UE for random access.

According to a fourth aspect of embodiments of the present invention,the above stated problem is solved by means of a UE for performing arandom access in a telecommunications system comprising a radio basestation to which, as mentioned above, is allocated a first set forming apool of non-dedicated random access preambles and a second set forming apool of dedicated random access preambles. The UE, according toembodiments of the present invention is configured to: receive a messagefrom the radio base station, the message comprising an identificationnumber of an available dedicated access preamble of the second set. Themessage further comprising information indicating in which of one ormore PRACH occurrences the dedicated random access preamble is valid tobe used by the UE for random access. The UE is further configured toperform a random access based on the received identification number ofthe available dedicated preamble and based on the indicated informationconcerning said one or more PRACH occurrences.

According to a fifth aspect of embodiments of the present invention, theabove stated problem is solved by means of a method in atelecommunications system, of assigning a preamble to a UE, for enablingthe UE to perform a random access. The system comprises a radio basestation that is allocated a first set forming a pool of non-dedicatedrandom access preambles and a second set forming a pool of dedicatedrandom access preambles. According to embodiments of the presentinvention, the method comprises the steps of: determining in the radiobase station, a dedicated random access preamble, of the second set,available for assignment to the UE; receiving at the UE, a message fromthe radio base station, the message comprises an identification numberof the available dedicated random access preamble and further comprisesinformation indicating in which of one or more PRACH occurrences thededicated random access preamble is valid to be used by the UE forrandom access; and performing, by the UE, a random access based on thereceived identification number of the available random access preambleand based on the indicated information concerning the one or more PRACHoccurrences.

According to a sixth aspect of embodiments of the present invention, theabove stated problem is solved by means of a telecommunications systemfor assigning a preamble to a UE, for enabling the UE to perform arandom access. The system comprises a radio base station that isallocated a first set forming a pool of non-dedicated random accesspreambles and a second set forming a pool of dedicated random accesspreambles. In the system, in accordance with embodiments of the presentinvention, the radio base station is configured to determine a dedicatedrandom access preamble, of the second set, available for assignment tothe UE; the UE is configured to receive a message from the radio basestation, the message comprises an identification number of the availablededicated random access preamble and further comprises informationindicating in which of one or more PRACH occurrences the dedicatedrandom access preamble is valid to be used by the UE for random access;and the UE is further configured to perform a random access based on thereceived identification number of the available random access preambleand based on the indicated information concerning the one or more PRACHoccurrences.

An advantage with the present invention is that the random access delayis reduced.

Another advantage with the present invention is that it allows tovirtually extend dedicated preambles by introducing a validity patternin the time domain such that to alleviate the case where there isshortage of dedicated preambles.

A further advantage with the present invention is that, for a given loadfor contention-free random access, the size of the set forming a pool ofdedicated preambles can be reduced. This in turn means that morenon-dedicated random access preambles are available, which implies thatthe risk for collisions is minimized.

Still other objects and features of the present invention will becomeapparent from the following detailed description in conjunction with theaccompanying drawings, attention to be called to the fact, however, thatthe following drawings are illustrative only, and that variousmodifications and changes may be made in the specific embodimentsillustrated as described within the scope of the appended claims. Itshould further be understood that the drawings are not necessarily drawnto scale and that, unless otherwise indicated, they are merely intendedto conceptually illustrate the structures and procedures describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a wireless networktelecommunications system wherein exemplary embodiments of the presentinvention can be applied.

FIG. 2 is a diagram illustrating a type 1 frame structure applicable toLTE FDD according to prior art.

FIG. 3A is a diagram illustrating an exemplary PRACH configuration usingan exemplary validity pattern applicable to exemplary embodiments of thepresent invention.

FIG. 3B is a diagram illustrating the exemplary PRACH configurationusing another exemplary validity pattern applicable to exemplaryembodiments of the present invention.

FIG. 3C is a diagram illustrating the exemplary PRACH configurationusing yet another exemplary validity pattern applicable to exemplaryembodiments of the present invention.

FIG. 4 is a diagram illustrating a flowchart of a method performed, in aradio base station, according to exemplary embodiments of the presentinvention.

FIG. 5 is a diagram illustrating a flowchart of a method, performed in auser equipment according to exemplary embodiments of the presentinvention.

FIG. 6 illustrates a block diagram of an exemplary radio base stationaccording to embodiments of the present invention.

FIG. 7 illustrates a block diagram of an exemplary user equipmentaccording to embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, scenarios, techniques, etc. in order to provide thoroughunderstanding of the present invention. However, it will be apparent tothe person skilled in the art that the present invention and itsembodiments may be practiced in other embodiments that depart from thesespecific details.

The different embodiments of the present invention are described hereinby way of reference to particular example scenarios. In particular, theinvention is described in a non-limiting general context in relation torandom access procedures in a telecommunications network system that isbased on the third generation (3G) long term evolution (LTE) concept. Itshould be noted that the present invention is not restricted to 3G LTEbut can be applicable in other wireless systems that employ randomaccess procedures, such as WiMAX (worldwide interoperability formicrowave access), or HSPA (high speed packet access) or HSUPA (highspeed uplink packet access) or HSDPA (high speed downlink packet access)or WCDMA (wideband code division multiple access) etc.

Referring to FIG. 1, there is illustrated a block diagram of anexemplary wireless telecommunications network system 100 in which thedifferent exemplary embodiment of the present invention may be applied.Note that the system depicted in FIG. 1 only shows transceivers or nodesthat are necessary for understanding the different exemplary embodimentsof the present invention. As shown, the system 100 which is hereconsidered to represent a simplified 3GPP LTE system, comprises a numberof user equipments UE 110, UE 111, UE 112, and apparatuses acting asradio base stations and denoted eNodeB 1 120, eNodeB 2 121 and eNodeB 3122. One of the functions of the eNodeBs is to control traffic to andfrom UEs in a cell. A UE is suitable to be used as a mobile phone, awireless terminal, a laptop, a personal computer, a personal digitalassistant, a voice over internet protocol (VoIP) capable phone or anyother 3GPP LTE capable equipment. Traffic, over a radio link, from aeNodeB to a UE is referred to as downlink (DL) traffic and traffic, overa radio link, from the UE to a eNodeB is referred to as uplink (UL)traffic. Note that in FIG. 1, the number of UEs and eNodeBs is onlyillustrative and the embodiments of the present invention are notrestricted to any particular number of UEs and/or number of eNodeBs.

Referring back to FIG. 1, it is here assumed that UE 111 and UE 113 areserved by eNodeB 1 120. In other words it is assumed that UE 111 and UE113 have already successfully random accessed the network 100 and areconnected wirelessly to the eNodeB 1 120. Furthermore, it is alsoassumed, for easily of understanding the principles of the embodimentsof the present invention, that a eNodeB e.g. eNodeB 1 120 is serving onecell alone, although this is not necessary. In LTE, there is in total aset comprised of 64 random access preambles available per cell (orconfigured for a cell). Thus, a eNodeB can be allocated/assigned these64 preambles. It should be noted that preambles assigned to differentcells controlled by the same eNodeB do not necessarily belong to thesame set of preambles i.e. different sets (and pools) are typically cellspecific. However, for better understanding the different embodiments ofthe present invention, it is here assumed that a eNodeB is serving asingle cell and therefore, in this case, it is adequate to state that aset of 64 preambles are assigned by eNodeB (or per cell).

A first set within this set of 64 preambles, forms a pool of preamblesfor use with contention-based random access. The preambles of this poolare, as mentioned earlier, known as non-dedicated random accesspreambles. This pool is primarily used when there is UE-originated dataand the UE has to establish a connection and/or an adequate uplinktiming relation with the network through the random access (RA)procedure. When performing contention-based random access, the UEchooses/selects a non-dedicated random access preamble from this firstset by random. For each non-dedicated random access preamble of thisfirst set is associated a preamble identification number (or preambleidentifier). Among the set of 64 preambles, there is also a second setforming a pool of dedicated random access preambles. A dedicated randomaccess preamble of the second set is, as mentioned before, used toperform a contention-free random access. For contention-free randomaccess, it is the eNodeB that assigns a dedicated random access preambleto the UE. In other words this type of random access is triggered by thenetwork (e.g. the eNodeB). For each dedicated random access preamble ofthis second set is also associated a preamble identification number (ID)(or preamble identifier). The eNodeB keeps track of the dedicatedpreamble IDs that are currently in use. For contention-free randomaccess, the eNodeB can therefore map the received preamble to the UEthat has sent the dedicated random access preamble and has tried toaccess the system or network system. Therefore, no contention resolutionprocedure needs to be performed.

Referring back to FIG. 1, assume that, in respect of the UE 110, acontention-free random access procedure is triggered by the network(e.g. by the eNodeB 1 120) so that the UE can establish a time alignmentand synchronization to the network (e.g. eNodeB). This is schematicallyillustrated using a dashed arrow between UE 110 and eNodeB 1 120. Asmentioned earlier, the eNodeB 1 120 keeps track of all the dedicatedrandom access preambles (and dedicated preamble IDs) it has allocatedand that are currently in use. Once a contention-free random access hassuccessfully completed or the validity of the dedicated preambleallocation has expired, the dedicated preamble can be re-used. However,in the event that none of the dedicated random access preambles, in thesecond set, is for the moment available for allocation to the UE 110,the eNodeB 1 120 has to refer UEs, for which there is no dedicatedpreamble, to perform a contention-based random access. However,contention-based random access generally leads to that potentialcollisions occur in addition to delays when a contention resolutionneeds to be performed.

Therefore, in order to alleviate an exemplary problem scenario that nodedicated preambles are available to perform a contention-free randomaccess, the eNodeB 1 120 in accordance with embodiments of the presentinvention, is configured to determine that a dedicated random accesspreamble from the second set is available for assignment to the UE 110and is further configured to transmit to the UE 110 a message, on e.g. aphysical downlink control channel (PDCCH), indicating the preamble ID ofthe determined dedicated random access preamble and further indicating avalidity pattern in e.g. the time domain specifying in which physicalrandom access channel (PRACH) occurrence the dedicated preamble is validto be used by the UE to conduct random access. As an example, since theeNodeB 1 120 keeps track of the dedicated preambles and associated IDsthat are currently in use, it also knows when a random access based on adedicated preamble has successfully completed or the validity of theallocation has expired and thus can determine that such a dedicatedpreamble is now available and can be re-used. If the eNodeB 1 120 needsto establish an uplink synchronization, it sends, in e.g. the PDCCHmessage, the preamble ID of this available preamble and informationindicating in which PRACH occurrence the dedicated preamble is valid forthe UE 110. The UE 110 then can perform a random access based on thepreamble ID received from the eNodeB 1 120 and also based the indicatedinformation concerning the PRACH occurrence(s).

It should be noted that in the LTE technical specification 3GPP TS36.211 entitled: “Evolved Universal Terrestrial Radio Access (E-UTRA)Physical Channels and Modulation (Release 8)”, downlink and uplinktransmissions are organised into radio frames wherein each radio framehas a duration of 10 ms. Two different radio frame structures aredefined, one frame structure known as type 1 which is applicable to FDD(Frequency Division Duplex) and another frame structure known as type 2applicable to TDD (Time Division Duplex). The above technicalspecification 3GPP TS 36.211 also defines that for FDD, 10 subframes perradio frame are available for downlink transmissions and 10 subframes(of the same radio frame) are available for uplink transmissions andthat uplink and downlink transmission are separated in the frequencydomain. Furthermore, a type 1 frame structure comprises 20 slots oflength 0.5 ms each and that 2 consecutive slots define a subframe whichis thus 1 ms long.

Referring to FIG. 2 there is illustrated a type 1 frame structureapplicable to LTE FDD, as defined in 3GPP TS 36.211. The technicalspecification 3GPP TS 36.211 also defines that PRACH resources and theiroccurrence(s) are configured for each cell (or eNodeB) and arecommunicated on either broadcast or dedicated control channels (e.g.PDCCH). Sixteen PRACH configurations are defined for FDD in 3GPP TS36.211.

According to an embodiment of the present invention and as mentionedearlier, the PDCCH message can convey the dedicated preamble IDdetermined as available by the eNodeB 1 120 and also convey theinformation indicating in which one or more PRACH occurrences, thededicated preamble is valid for the UE. As an example, the informationcan indicate in which subframe(s) the assigned dedicated random accesspreamble is valid to be used by the UE. According to an embodiment ofthe present invention, the subframe(s) is/are tied to the framestructure e.g. the radio frame structure.

In the following, an exemplary embodiment shall be described in moredetail in the context of FDD PRACH configuration 12. PRACH configuration12 defines that PRACH resources occur in subframe #0; subframe #2,subframe #4, subframe #6 and subframe #8 of each radio frame.

As an exemplary embodiment of the present invention, the information inthe PDCCH message can indicate in a field of said PDCCH message, thate.g., in case of PRACH configuration 12, the available dedicatedpreambles is valid only in subframe #2 in each radio frame. This isillustrated in FIG. 3A where it is indicated that the dedicated preambleis valid for the particular UE in subframe 2 in each frame. It should bementioned that for PRACH configuration 12, there are, as describedabove, several subframes that can be selected where the dedicatedpreamble is valid for the UE. Such subframes are also shown in FIG. 3Aand are numbered, as above, namely, subframe #0; subframe #2, subframe#4, subframe #6 and subframe #8. However, the example described andillustrated in FIG. 3A shows that the dedicated preamble is valid onlyin subframe #2 in each radio frame, in accordance with the informationindicated in the PDCCH message. It should be noted that it is alsoimplied here that e.g. the same dedicated preamble can be assigned todifferent UEs for subframe #0, subframe #4, subframe #6, and/or subframe#8 respectively. Effectively, this means that the dedicated preambleavailability is increased in this example by a factor of 5. Obviously,the usage of such a validity pattern significantly reduces the risk ofrunning out of dedicated preambles that are available for assignment.

It should be mentioned that the exemplary embodiments of the presentinvention are not restricted to PRACH configuration 12. In other wordsall PRACH configurations defined for FDD (e.g. frame structure type 1)can be used. As an example, a conceivable configuration for PRACHresources could be that they occur in all radio frames in subframe #1(PRACH configuration 3).

Another example using PRACH configuration 12, is that the PRACHresource, for the UE, occurs, in each frame, in subframe #0; subframe#2, subframe #4, subframe #6 and subframe #8. This is shown in FIG. 3Bwhere subframe #0; subframe #2, subframe #4, subframe #6 and subframe #8are indicated inside each frame respectively. The subframes that are notused have a white background (e.g. subframe #7, subframe #1, etc . . .).

As mentioned above, for PRACH configuration 12, there are severalsubframes for which the dedicated preamble can be valid for the UE. Suchsubframes are subframe #0; subframe #2, subframe #4, subframe #6 andsubframe #8. Therefore, according to another exemplary embodiment of thepresent invention, the field in the PDCCH message for dedicated preambleassignment can indicate the periodicity for which of the PRACHoccurrence(s) the dedicated preamble is valid to be used by the UE forrandom access. For example, the field could indicate that for everyfourth PRACH occurrence the dedicated preamble is valid for the UE. Thisis illustrated in FIG. 3C where it is shown that, for PRACHconfiguration 12, the subframe sequence (periodicity) is e.g. subframe#8 (of frame 1), subframe #6 (of frame 2), subframe #4 (of frame 3),subframe #2 (of frame 4 (not shown)) etc . . . during which thededicated random access preamble is valid for the UE to perform a randomaccess. Thus, in this example a dedicated preamble is valid in everyfourth PRACH occurrence. In FIG. 3C, the subframes of the sequence areshown with a cross inside.

Note that in some configurations, such as PRACH configuration 12, it ise.g. not useful for a UE to use subsequent PRACH resources since theyfollow each other too closely. The UE needs to wait for a response fromthe eNodeB (or network) before it is allowed to repeat a random accessattempt. Thus, it is not useful for the UE to re-use, e.g., all of thePRACH resources of configuration 12.

The embodiments of the present invention thus increase the availabilityof dedicated preambles by defining a validity pattern in the time domainfor the LTE FDD contention-free access. Note however that the sameprinciple is also applicable for PRACH configurations defined for LTETDD (i.e. frame structure type 2). Thus, the embodiments of the presentinvention are not restricted to LTE FDD. It should be noted that in LTETDD a field in the PDCCH message can indicate for which PRACHresource(s) in the frequency domain and/or the time domain the dedicatedpreamble is valid.

According to another embodiment of the present invention, theinformation transmitted by the eNodeB in the message (e.g. PDCCHmessage) including the dedicated preamble ID, indicates/specifies abinary value indicative of one or more PRACH occurrences. This binaryvalue is expressed by a predetermined number of bits indicated in themessage (e.g. by 3 bits or 4 bits). The value can be either an explicitvalue or an input value to a function giving the value(s) or an indexfor a table or an index to a table entry or an index in a table thatcontains said value(s). As an exemplary embodiment of the presentinvention, the binary value may correspond to a PRACH index (e.g. a maskindex) being associated with a predefined PRACH configuration indicatingin which of one or more allowed PRACH occurrences, the dedicatedpreamble is valid to be used by the UE for random access. Note that by aPRACH configuration is meant e.g. a PRACH configurationindicated/defined in the previously described technical specification3GPP 36.211. It should be noted that the PRACH index value determinesthe validity of the indicated dedicated preamble. As an exemplaryembodiment of the present invention, the binary value may correspond toa PRACH index (e.g. a mask index). In association with the configuredPRACH configuration, defined in the previously described technicalspecification 3GPP 36.211, the PRACH index determines the one or moresubframes of a frame in which the assigned dedicated preamble is valid.For example, the FDD PRACH configuration 12 corresponds to PRACHresources in subframe #0, subframe #2, subframe #4, subframe #6 andsubframe #8. The PRACH index determines for which one or more subframesof the above listed subframes the dedicated preamble is valid.

As a further example, in a case where all of the preambles in the poolof dedicated random access preambles are already allocated, the eNodeBis configured to determine when a random access based on a dedicatedpreamble has successfully completed or the validity of the allocationhas expired, and then re-use the available dedicated preamble andtransmit to the UE the preamble ID (of the available dedicated preamble)in the PDCCH message and indicate in that message a PRACH mask index.The UE then receives such a PDCCH message and performs a random accessbased on the preamble ID and based on the PRACH mask index since thePRACH mask index, as described above, is associated with a predefinedconfiguration indicating in which allowed PRACH occurrences the preambleis valid to be used by the UE.

As an example, if the eNodeB explicitly signals in e.g. the PDCCHmessage, to the UE a dedicated preamble ID having a fixed ID value, andthe eNodeB further indicates in the message, the PRACH index (e.g. abinary value corresponding to a PRACH index (e.g. a mask index)) that isused by the UE to point at one or more subframes in which the dedicatedpreamble is valid (i.e. to point to a validity pattern which describesin which PRACH occurrences the dedicated preamble is valid for use bythe UE), then the UE knows, based on the dedicated preamble ID and thePRACH index, in which PRACH occurrence(s) the dedicated preamble isvalid to it to perform a random access using the dedicated random accesspreamble.

It should be noted that the binary value (e.g. 3 bits or 4 bits) thatassigns an offset, relative to a time reference, of the validity patternof the PRACH resource could be an explicit value, input to a functiongiving the values or an index into a table that contains the values.Alternatively, a joint encoding of the binary value in the field thatassigns the validity pattern of the PRACH resource and the binary valuein the field that assigns the offset, relative to a time reference, ofthe validity pattern of the PRACH resource could be an explicit value,input to a function giving the values or an index into a table thatcontains the values.

Referring to FIG. 4 there is illustrated a flowchart of a method aimedto be performed/implemented in a radio base station (e.g. eNodeB), inaccordance with the above described embodiments of the presentinvention. As mentioned earlier, the radio base station is assigned afirst set forming a pool of non-dedicated random access preambles and asecond set forming a pool of dedicated random access preambles. Themethod in the radio base station enables a UE to perform random access.The method, in the radio base station, comprises the following mainsteps:

-   (401); determining a dedicated random access preamble from the    second set available for assignment to the UE;-   (402) transmitting a message (e.g. on a PDCCH message or a field on    the PDCCH message) to the UE, the message comprising the dedicated    preamble ID of the available dedicated preamble that was determined    by the radio base station, and further comprising information in    which of one or more PRACH occurrences the dedicated random access    preamble is valid to be used by the UE for random access.

As mentioned before, the information in the PDCCH message indicates inwhich subframe(s) the assigned dedicated preamble is valid to be used bythe UE. The one or more subframes are tied to a frame structure, aspreviously described. The information may also indicate to the UE, theperiodicity for which of the PRACH occurrence(s) the dedicated preambleis valid for the UE. Furthermore, the information can be indicated usinga binary value, said binary value is expressed by a predefined number ofbits. (i.e. 4 bits). As mentioned before, the binary value maycorrespond to a PRACH index that is associated with a predefinedconfiguration indicating in which of allowed PRACH occurrence(s), thededicated preamble is valid to be used for the UE.

Referring to FIG. 5 there is illustrated a flowchart of a method aimedto be performed/implemented in a UE, in accordance with the previouslydescribed embodiments of the present invention. The method in the UE,enables said UE to perform a random access in a system wherein a radiobase station is provided; said base station being assigned a first setforming a pool of non-dedicated random access preamble and a second setforming a pool of dedicated random access preambles. The method, to beperformed in the UE, comprises the following main steps:

-   (501) receiving a message from the radio base station (or eNodeB),    said message (on e.g. the PDCCH) comprises a preamble ID of an    available dedicated random access preamble of the second set and    further comprising information indicating in which of one or more    PRACH occurrences the dedicated preamble is valid to be used by the    UE for random access;-   (502) performing a random access procedure based on the preamble ID    of the available dedicated preamble and based on the indicated    information concerning the one or more PRACH occurrences.

The different exemplary embodiments describing the subframes; theperiodicity, the binary value etc. have already been described and aretherefore not unnecessarily repeated again.

Referring to FIG. 6 there is illustrated a block diagram of an exemplaryradio base station 600 (e.g. eNodeB) being configured/arranged to assignto a UE a dedicated random access preamble, said radio base stationbeing allocated a first set forming a pool of non-dedicated randomaccess preambles and a second set forming a pool of dedicated randomaccess preambles. As shown in FIG. 6, the radio base station 600comprises processing means 601 adapted to determine a dedicated randomaccess preamble, from the second set, available for assignment to theUE. As an example, once a random access based on a dedicated preamblehas successfully completed or the validity of the allocation hasexpired, an allocated dedicated preamble can be re-used. The radio basestation 600 is configured to transmit, by means of transmitting means602A, denoted TX in FIG. 6, a message to the UE, on e.g. the PDCCHmessage, the dedicated and available preamble ID and further configuredto indicate in that message information indicating in which of one ormore PRACH occurrences the dedicated preamble is valid to be used by theUE for random access. As shown in FIG. 6, the radio base station 600further comprises receiving means 602B, denoted RX in FIG. 6, which isarranged/configured to receive from the UE the assigned dedicatedpreamble that the UE used/uses to perform random access based on thededicated preamble ID and also based on the indicated informationconcerning the one or more PRACH occurrences. Additional actionsperformed by the radio base station have already been described and aretherefore not repeated. It should however be mentioned that thedifferent exemplary blocks shown in FIG. 6 are not necessarilyseparated. Furthermore the TX means 602A and the RX means 602B are notnecessarily included in the same block i.e. they each can be representedby a single block. The radio base station 600 is therefore notrestricted and is not limited to the exemplary block diagrams shown inFIG. 6. In addition, the radio base station 600 may also comprise otherelement and/or blocks not illustrated in FIG. 6.

Referring to FIG. 7 there is illustrated a block diagram of an exemplaryuser equipment (UE) 700 capable in performing a random access using anassigned dedicated random access preamble, in accordance with previouslydescribed embodiments of the present invention. As shown in FIG. 7, theUE 700 comprises receiving means 701A, denoted RX in FIG. 7, which isadapted/configured to receive from a radio base station a messagecomprising a dedicated preamble ID and further comprising informationindicating in which of at least one PRACH occurrence the dedicatedpreamble is valid to be used by the UE. As a non-limiting example, themessage is received on a PDCCH. The UE 700 further comprisestransmitting means 701B, denoted TX in FIG. 7, which isadapted/configured to transmit, based on the received dedicated preambleID and on the indicated information concerning the one or more PRACHoccurrence, the dedicated random access preamble in order to perform arandom access. The UE 700 also further comprises processing means 702 toe.g. process one or more received/transmitted messages. Again, thedifferent exemplary embodiments describing the subframes; theperiodicity, the binary value (e.g. 4 bits), etc. have already beendescribed and are therefore not unnecessarily repeated.

As described above, several advantages are achieved by the differentembodiments of the present invention. As an example, the random accessdelay is reduced even in cases where all of the preambles in the pool ofdedicated random access preambles are already in use. Another advantagewith the different exemplary of the present invention is that it allowsto virtually extend dedicated preambles by introducing, as mentionedearlier, a validity pattern in the time domain such that to alleviatethe case where there is shortage of dedicated preambles. Yet anotheradvantage with the present invention is that for each random access thatdoes not need to be referred to the usage of non-dedicated random accesspreambles, random access delays can be reduced.

The present invention and its embodiments can be realized in many ways.For example, one embodiment of the present invention includes acomputer-readable medium having instructions stored thereon that areexecutable by a radio base station (e.g. eNodeB or eNB) and/or a UE of atelecommunications system. The instructions executable by the radio basestation and/or the UE and stored on a computer-readable medium performthe method steps of the present invention as set forth in the claims.

While the invention has been described in terms of several preferredembodiments, it is contemplated that alternatives, modifications,permutations and equivalents thereof will become apparent to thoseskilled in the art upon reading of the specifications and study of thedrawings. It is therefore intended that the following appended claimsinclude such alternatives, modifications, permutations and equivalentsas fall within the scope of the present invention.

The invention claimed is:
 1. A method in a radio base station ofassigning a preamble to a user equipment (UE), the method comprising:determining, from a set of dedicated random access preambles, a randomaccess preamble to assign to the UE; determining a periodicallyreoccurring physical random access channel (PRACH); and transmitting amessage to the UE, the message comprising an identification number ofthe determined random access preamble and further comprising informationindicating a periodically reoccurring PRACH occurrence during which thedetermined random access preamble is valid to be used by the UE forrandom access.
 2. The method according to claim 1, wherein determiningthe periodically reoccurring PRACH includes determining a specificsubframe of a radio frame for the reoccurring PRACH to reoccur and themessage transmitted to the UE includes the specific subframe of theradio frame.
 3. The method according to claim 2, wherein a differentspecific subframe can be used for different PRACH configurations.
 4. Themethod according to claim 1, wherein determining the periodicallyreoccurring PRACH includes determining specific subframes of a radioframe for the reoccurring PRACH to reoccur and the message transmittedto the UE includes the specific subframes of the radio frame.
 5. Themethod according to claim 4, wherein different specific subframes can beused for different PRACH configurations.
 6. The method according toclaim 1, wherein the information indicating the periodically occurringPRACH occurrence includes a field with a binary value indicative of theperiodically occurring PRACH occurrence.
 7. The method according toclaim 6, wherein the binary value is expressed by a predefined number ofbits.
 8. The method according to claim 1, wherein the transmitting themessage to the UE includes transmitting the message on a physicaldownlink control channel (PDCCH).
 9. The method according to claim 1,wherein the periodically reoccurring PRACH occurrence is indicated froma set of periodically reoccurring PRACH occurrences.
 10. A radio basestation for assigning a preamble to a user equipment (UE), the radiobase station comprising: a processor configured to determine, from a setof dedicated random access preambles, a random access preamble to assignto the UE; the processor configured to determine a periodicallyreoccurring physical random access channel (PRACH); and a transmitterconfigured to transmit a message to the UE, the message comprising anidentification number of the determined random access preamble andfurther comprising information indicating a periodically reoccurringPRACH occurrence during which the determined random access preamble isvalid to be used by the UE for random access.
 11. The radio base stationaccording to claim 10, wherein to determine the periodically reoccurringPRACH includes determining a specific subframe of a radio frame for thereoccurring PRACH to reoccur and the message transmitted to the UEincludes the specific subframe of the radio frame.
 12. The radio basestation according to claim 11, wherein a different specific subframe canbe used for different PRACH configurations.
 13. The radio base stationaccording to claim 10, wherein to determine the periodically reoccurringPRACH includes determining specific subframes of a radio frame for thereoccurring PRACH to reoccur and the message transmitted to the UEincludes the specific subframes of the radio frame.
 14. The radio basestation according to claim 13, wherein different specific subframes canbe used for different PRACH configurations.
 15. The radio base stationaccording to claim 10, wherein the information indicating theperiodically occurring PRACH occurrence includes a field with a binaryvalue indicative of the periodically occurring PRACH occurrence.
 16. Theradio base station according to claim 15, wherein the binary value isexpressed by a predefined number of bits.
 17. The radio base stationaccording to claim 10, wherein to transmit the message to the UEincludes transmitting the message on a physical downlink control channel(PDCCH).
 18. The radio base station according to claim 10, wherein theperiodically reoccurring PRACH occurrence is indicated from a set ofperiodically reoccurring PRACH occurrences.
 19. A method of enabling auser equipment (UE) to perform a random access in a telecommunicationssystem, the method comprising: receiving a message from a radio basestation, the method comprising an identification number of a dedicatedrandom access preamble assigned to the UE, the message furthercomprising information indicating a periodically reoccurring PRACHoccurrence during which the determined random access preamble is validto be used by the UE for random access; and performing a random accessbased on the received identification number and based on the informationindicating the periodically reoccurring PRACH occurrence.
 20. The methodaccording to claim 19, wherein the message includes a specific subframeof a radio frame for the reoccurring PRACH to reoccur.
 21. The methodaccording to claim 20, wherein a different specific subframe can be usedfor different PRACH configurations.
 22. The method according to claim19, wherein the message includes specific subframes of the radio frame.23. The method according to claim 22, wherein different specificsubframes can be used for different PRACH configurations.
 24. The methodaccording to claim 19, wherein the information indicating theperiodically occurring PRACH occurrence comprises a field with a binaryvalue indicative of the periodically occurring PRACH occurrence.
 25. Themethod according to claim 24, wherein the binary value is expressed by apredefined number of bits.
 26. The method according to claim 19, whereinreceiving the message from the radio base station includes receiving themessage on a physical downlink control channel (PDCCH).
 27. The methodaccording to claim 19, wherein the periodically reoccurring PRACHoccurrence is indicated from a set of periodically reoccurring PRACHoccurrences.
 28. A user equipment (UE) enabled to perform a randomaccess in a telecommunications system, the UE comprising: a receiverconfigured to receive a message from a radio base station, the methodcomprising an identification number of a dedicated random accesspreamble assigned to the UE, the message further comprising informationindicating a periodically reoccurring PRACH occurrence during which thedetermined random access preamble is valid to be used by the UE forrandom access; and a processor configured to perform a random accessbased on the received identification number and based on the informationindicating the periodically reoccurring PRACH occurrence.
 29. The UEaccording to claim 28, wherein the message includes a specific subframeof a radio frame for the reoccurring PRACH to reoccur.
 30. The UEaccording to claim 29, wherein a different specific subframe can be usedfor different PRACH configurations.
 31. The UE according to claim 28,wherein the message includes specific subframes of the radio frame. 32.The UE according to claim 31, wherein different specific subframes canbe used for different PRACH configurations.
 33. The UE according toclaim 28, wherein the information indicating the periodically occurringPRACH occurrence comprises a field with a binary value indicative of theperiodically occurring PRACH occurrence.
 34. The UE according to claim33, wherein the binary value is expressed by a predefined number ofbits.
 35. The UE according to claim 28, wherein the message from theradio base station includes receiving the message on a physical downlinkcontrol channel (PDCCH).
 36. The UE according to claim 28, wherein theperiodically reoccurring PRACH occurrence is indicated from a set ofperiodically reoccurring PRACH occurrences.